Hydroprocessing can include processes which convert hydrocarbons in the presence of hydroprocessing catalyst and hydrogen to more valuable products. Hydrocracking is a hydroprocessing process in which hydrocarbons crack in the presence of hydrogen and hydrocracking catalyst to lower molecular weight hydrocarbons. Depending on the desired output, a hydrocracking unit may contain one or more fixed beds of the same or different catalyst. Hydrotreating is a process in which hydrogen is contacted with a hydrocarbon stream in the presence of hydrotreating catalysts which are primarily active for the removal of heteroatoms, such as sulfur, nitrogen and metals from the hydrocarbon feedstock. In hydrotreating, hydrocarbons with double and triple bonds may be saturated. Aromatics may also be saturated. Some hydrotreating processes are specifically designed to saturate aromatics.
Two-stage hydrocracking processes involve fractionation of a hydrocracked stream from a first stage hydrocracking reactor followed by hydrocracking of an unconverted oil (UCO) stream in a second stage hydrocracking reactor. However, the best two-stage hydrocracking process cannot achieve full conversion to materials boiling below the diesel cut point. Typically, a bottoms stream from the fractionation column in two-stage hydrocracking comprises a recycle oil (RO) stream and an UCO stream. The RO is recycled to the second stage hydrocracking reactor while the UCO is purged from the process to remove unconvertible heavy polynuclear aromatics (HPNA's) from the process. HPNA's are fused aromatic rings comprising more than eight rings. HPNA's in RO and UCO can cause significant adverse impact on hydrocracking operations such as fouling of the exchangers and coking on the catalyst. Several processes are available to manage HPNA rejection, such as steam stripping and adsorption.
Better processes and apparatuses are needed to remove HPNA's from RO streams and to improve hydrocracking conversion.